The hydrolysis reactions of many complex metal hydrides, including sodium borohydride (NaBH4), have been commonly used for the generation of hydrogen gas. The governing chemical reaction may be expressed as:
where MBH4 and MBO2 respectively represent a metal borohydride and a metal metaborate. The hydrolysis of sodium borohydride is typically slow at room temperature and heat or a catalyst, e.g., acids, a variety of transition metals, such as ruthenium, cobalt, nickel, or iron, or corresponding metal borides in solution or deposited on inert supports or as solids, can be used to accelerate the hydrolysis reaction. In addition, the rate of decomposition of the complex metal hydride into hydrogen gas and a metal metaborate is pH dependent, with higher pH values hindering the hydrolysis. Accordingly, solutions of a complex metal hydride, such as sodium borohydride, a stabilizer, such as sodium hydroxide (NaOH), and water are used as the fuel, i.e., the consumable element, from which the hydrogen gas is generated. To expedite the production of the hydrogen gas, the fuel is passed over a catalyst. The output of this process is hydrogen gas and a discharged fuel solution. When the complex metal hydride is sodium borohydride, the discharged fuel is a slurry of sodium metaborate. To meet the demands of commercial applications, most hydrogen generating systems also store the fuel and such storage gives rise to several disadvantages. One disadvantage arises from the presence of the stabilizer. The function of the stabilizer is to raise the pH value of the fuel solution and, thereby prevent the hydrolysis until the solution contacts the catalyst. As the stabilizer does not participate in any chemical reaction, both the fuel and discharged fuel solutions have a high pH value. Typically, both the fuel and discharged fuel solutions have pH values between 13 and 14. This high pH requires that the transport of both the fuel and discharged fuel solutions comport with governmental regulations which would increase the cost of hydrogen generation. The presence of these high pH solutions is also an impediment to the commercialization and public acceptance of the process. Additional costs are imposed by the presence of these high pH solutions as they react with a variety of metals. To avoid these reactions, non-reactive materials, such as stainless or non-reactive plastics, must be used in the hydrogen generation system.
While solid complex metal hydrides in a variety of forms, including powder, pellets and granules, are manufactured for pharmaceutical applications, their use in commercial systems for the controlled and measured generation of hydrogen has not been provided.
Based on the foregoing, it would be extremely desirable if a hydrogen generation system could be devised which meets the needs of commercial applications and which overcomes the problems associated with the use of premixed fuel solutions.